The Portugal attacker was forced off just nine minutes into Wednesday's 1-1 draw at Valladolid with a suspected thigh problem and will not be risked at the Balaidos, with the capital club's Champions League final against city rivals Atletico just a fortnight away.
"Cristiano won't play because he isn't at 100 per cent," Ancelotti told reporters on Saturday.
"I've always said we won't risk it because Cristiano isn't at his 100 per cent. We have to win without him."
However, Ancelotti confirmed that Sami Khedira, who has been out of action since suffering knee ligament damage while on international duty with Germany in November, is now ready to make his eagerly-awaited comeback after being an unused substitute for Madrid's two previous games.
"Khedira is fine," the Italian coach enthused. "He hasn't had any problems and can't wait to play."
Madrid currently sit third in La Liga, four points behind leaders Atletico, and must defeat Celta if they are to keep their slim title hopes alive.